Darkening/Lightening camera change effect
Assuming Cycles renderer, the Film panel in the Render properties includes an Exposure setting. The Exposure effectively controls how much "light" is allowed into the camera.

Simply keyframe the Exposure from 1.0 to 0.0 in the few frames before your camera change and then keyframe back from 0.0 to 1.0 after, resulting in a fade to black on the 'old' view and fade from black for the 'new'.